The field of harmonic conversion of laser light contains many candidate materials for harmonic conversion. It is estimated that there may be in excess of 100,000 candidate crystalline materials to choose from in testing for suitability in laser harmonic conversion applications. However, as testing takes place deficiencies are usually detected in one property or another. A good material should efficiently convert the laser light, should be machinable, should be minimally hydroscopic if at all, and should be capable of relatively fast crystal growth.
Efficient frequency conversion can occur in several ways. These ways are referred to in the laser art as Type I and Type II harmonic conversion and as Type I, Type II and Type III sum frequency and difference frequency mixing. Each is well known in the laser art as shown by the references "Phase-Matched Second-Harmonic Generation in Biaxial Crystals", by M. V. Hobden, Journal of Applied Physics, Vol. 38, No. 11, October 1967; "Generalized Study on Angular Dependence of Induced Second-order Optical Polarizations and Phase Matching in Biaxial Crystals" by Hiromasa Ito, Journal of Applied Physics, Vol. 46, No. 9, September 1975; and Introduction to Optical Electronics by Amnon Yariv (Holt, Rinehart and Winston, Inc., 1971) at pages 189 to 198.
Crystal growth techniques suitable for making harmonic conversion crystals are well known in the art. The crystals can be grown in solution or by use of a seed crystal. Examples are contained in the book R. A. Landise "The Growth of Single Crystals" (Prentice Hall 1970), Chapter 7.
The current standard in the harmonic conversion crystal field for converting laser light in the one micron region is potassium dihydrogen phosphate (KDP). KDP is marginally hydroscopic, and it has a damage threshold of 7 Joules/cm.sup.2. Thus KDP degrades due to exposure to the atmosphere and can be one of the first optical components to damage in a high power laser. For these reasons, a long-term, multi-year search was undertaken at Lawrence Livermore National Laboratory (LLNL) and by others in the art to find a substitute material for KDP. This effort revealed the literature reference "L-Arginine Phosphate Monohydrate" by Katsuyuki Aoki, et al., Acta Cryst. (1971) B27, 11. and "A New Phase Matchable Nonlinear Optic Crystal L-Abginne Phosphate Monohydrate", Acta Chimica Sinica, Vol. 41, No. 6 (June 1983) by Xu Dong, et al.
Aoki, et al. describe the crystalline structure of 1-arginine phosphate monohydrate (LAP). Dong, et al. reports that their crystal was placed in a laser beam to confirm that LAP would frequency double and sum frequency match. However, this prior work did not involve testing of the damage threshold of the LAP crystal, and even more importantly the prior LAP studies missed the 1.05 micron absorption band. Also, no further work has been found on detailed analysis to confirm the properties of LAP as a frequency converter. As part of the LLNL program in the area, LAP has been grown and characterized. The results reveal that LAP has excellent characteristics as a replacement for KDP except in one crutial aspect. LAP has an absorption band at one micron. Such absorption makes LAP useless in most harmonic conversion applications, which occur at one micron wavelength.